Three species of Chlamydia cause human disease, including sexually transmitted diseases and pneumonias. Most are susceptible to azithromycin, doxycycline, and some fluoroquinolones.

Chlamydiae are nonmotile, obligate intracellular organisms. Although originally considered viruses because they require a cellular host, they are now known to be bacteria; they contain DNA, RNA, and ribosomes and make their own proteins and nucleic acids. However, because they synthesize most of their own metabolic intermediates, they cannot make their own ATP and thus are energy parasites.

Three species cause human disease:

• Chlamydia trachomatis
• Chlamydophila (formerly Chlamydia) pneumoniae
• Chlamydophila (formerly Chlamydia) psittaci

C. trachomatis has 18 immunologically defined serovars. Serovars A, B, Ba, and C cause trachoma and inclusion conjunctivitis; D through K cause sexually transmitted diseases (STDs) localized to mucosal surfaces; L1, L2, and L3 cause STDs that lead to invasive lymph node disease (lymphogranuloma venereum). In the US, C. trachomatis is the most common bacterial cause of STDs, including nongonococcal urethritis (see Sexually Transmitted Diseases (STDs): Chlamydial, Mycoplasmal, and Ureaplasmal Infections) and epididymitis in men; cervicitis, urethritis, and pelvic inflammatory disease in women; and proctitis, lymphogranuloma venereum, and reactive arthritis (Reiter's syndrome) in both sexes. Maternal transmission of C. trachomatis causes neonatal conjunctivitis and pneumonia. The organism is occasionally isolated from the throat in adults but rarely causes symptomatic pharyngitis.

C. pneumoniae can cause pneumonia (especially in children and young adults) that may be clinically indistinguishable from pneumonia caused by Mycoplasma pneumoniae. In some patients with C. pneumoniae, pneumonia, hoarseness, and sore throat may precede coughing, which may be persistent and complicated by bronchospasm. From 6 to 19% of community-acquired pneumonia cases are due to C. pneumoniae, but chlamydial pneumonia is uncommon among children < 5 yr. No seasonal variations in occurrence have been observed. The organism has been found in atheromatous lesions, and infection may be associated with increased risk of coronary artery disease, although proof of a connection has not yet been established.

C. psittaci causes psittacosis. Strains causing human disease are usually transmitted from psittacine birds (eg, parrots), causing a disseminated disease characterized by pneumonitis.

Diagnosis

• Clinical evaluation
• Sometimes nucleic acid–based testing

The diagnosis is sometimes made without testing (eg, in men with typical nongonococcal urethritis). However, because many cases are asymptomatic, especially in women, routine testing for genital infection has been recommended and is increasingly common. In cases of urethritis, diagnosis is often made by excluding gonorrhea as a cause or by presuming that both chlamydial infection and gonorrhea are present.

C. trachomatis can be isolated by diagnostic cell culture but is best identified in genital samples using nucleic acid amplification tests (NAATs) such as PCR because these tests are more sensitive than cell culture and have less stringent sample handling requirements. NAATs for genital infection can be done using noninvasively obtained samples, such as urine or vaginal swabs obtained by the patient or clinician. An enzyme-linked immunosorbent assay (ELISA) or a direct immunofluorescent slide test can detect antigens in genital and ocular infections, but both are less sensitive than culture or NAATs. Serologic tests are useful in diagnosing pneumonia in infants and lymphogranuloma venereum.

A primary clue to diagnosis of C. psittaci infection is close contact with birds, typically parrots or parakeets.

Screening: Because chlamydial genital infection is so common and often causes mild or nonspecific symptoms (particularly in women), routine screening of asymptomatic people at high risk of STDs is recommended. People who should be screened include

• People with a history of a previous STD
• People with high-risk behaviors
• Sexually active adolescents and young adults < 24 yr
• Pregnant women < 24 yr

(See also the US Preventive Services Task Force's summary of recommendations regarding screening for chlamydial infection.)

Treatment

• AzithromycinSome Trade Names
  ZITHROMAX
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  or doxycyclineSome Trade Names
  PERIOSTAT
  VIBRAMYCIN
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Uncomplicated lower genital tract infection is typically treated with a single dose of azithromycinSome Trade Names
ZITHROMAX
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(1 g po) or with a 7-day regimen of doxycyclineSome Trade Names
PERIOSTAT
VIBRAMYCIN
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(100 mg po bid) or some fluoroquinolones (eg, levofloxacinSome Trade Names
IQUIX
LEVAQUIN
QUIXIN
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500 mg po once/day). Treatment of presumed chlamydial infection is routine when gonorrhea is present (see also Sexually Transmitted Diseases (STDs): Gonorrhea). Pelvic inflammatory disease, lymphogranuloma venereum, or epididymitis is usually treated for 2 wk.

Specific infections are discussed elsewhere in The Manual: Psittacosis and C. pneumoniae pneumonia on Pneumonia: Etiology, lymphogranuloma venereum and urethritis on Sexually Transmitted Diseases (STDs): Lymphogranuloma Venereum (LGV), epididymitis on Penile and Scrotal Disorders: Epididymitis, reactive arthritis on Joint Disorders: Reactive Arthritis, neonatal conjunctivitis and neonatal pneumonia on Infections in Neonates: Neonatal Conjunctivitis and Infections in Neonates: Neonatal Pneumonia, and trachoma and inclusion conjunctivitis on see Conjunctival and Scleral Disorders: Trachoma.

Last full review/revision December 2009 by Byron E. Batteiger, MD

Content last modified February 2012